看到求回文串就想到 。
因为只有 所以判断条件改成等于。
不过这道题满足题意的串长度一定为偶，所以不能走到 根。
其他的就是板子了。
#include <cstdio>
#include <cstring>
#include <iostream>
using namespace std;
const int MAXN = 500000 , MAXK = 2;
int n;
char str[ MAXN + 5 ];
struct Palindrome_Automaton{
int Size , Last , Root0 , Root1 , Trans[ MAXN + 5 ][ MAXK + 5 ] , Link[ MAXN + 5 ];
int Len[ MAXN + 5 ] , Cnt[ MAXN + 5 ];
Palindrome_Automaton( ) {
Root0 = Size ++ , Root1 = Size ++; Last = Root0;
Len[ Root0 ] = 0 , Link[ Root0 ] = Root1;
Len[ Root1 ] = -1 , Link[ Root1 ] = Root1;
}
void Extend( int ch , int dex ) {
int u = Last;
for( ; u != 1 && ( str[ dex - Len[ u ] - 1 ] == str[ dex ] || ( dex - Len[ u ] - 1 == -1 ) ) ; u = Link[ u ] );
if( str[ dex - Len[ u ] - 1 ] == str[ dex ] || ( dex - Len[ u ] - 1 == -1 ) ) { Last = 0; return; }
if( !Trans[ u ][ ch ] ) {
int Newnode = ++ Size , v = Link[ u ];
Len[ Newnode ] = Len[ u ] + 2;
for( ; v != 1 && ( str[ dex - Len[ v ] - 1 ] == str[ dex ] || ( dex - Len[ u ] - 1 == -1 ) ) ; v = Link[ v ] );
Link[ Newnode ] = str[ dex - Len[ u ] - 1 ] == str[ dex ] ? 0 : Trans[ v ][ ch ];
Trans[ u ][ ch ] = Newnode;
Cnt[ Newnode ] = Cnt[ Link[ Newnode ] ] + 1;
}
Last = Trans[ u ][ ch ];
}
long long Build( char *str ) {
int len = strlen( str );
long long Ans = 0;
for( int i = 0 ; i < len ; i ++ ) {
Extend( str[ i ] - '0' + 1 , i );
Ans += Cnt[ Last ];
}
return Ans;
}
}PAM;
int main() {
scanf("%d",&n);
scanf("%s", str );
printf("%lld", PAM.Build( str ) );
return 0;
}